What is the best primer to use before painting?

There is an excellent article in the latest issue of FSM on this point. Any paint that adheres well to the material can be used as a primer. And it doesn't have to be white or gray. It can be the mid-range color of the finished piece.  

general rule is to stay with the same system that you started from primer 

this meaning, an enamel primer then enamel base coat, lacquer primer then lacquer base coat, acrylic primer etc 

with that said, for a styrene IM model part, enamel (Krylon, Rusto, generic econo brand aerosol as mentioned above) primer will work 

if you use an acrylic lacquer (Dupli-Color, PlastiKote, other automotive products, etc) then spray several light mist coats and sanding between until coverage is satisfactory 

any of these aerosol can primers can be sprayed into it's own can cap, then poured into AB cup

Primers are essentially all the same, just the name on the label is different. Sure their are minor differences but those don't effect the paint .

Or you could just buy a rattle can at Wal-Mart. It's cheap, and works just as well, if not better then anything you might buy specifically for airbrush use.